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      <h3><a href="../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3>
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      <h1 align="center">Serialization</h1>
      <h2 align="center"><code>smart_cast</code></h2>
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<h3>Motivation</h3>
To cast from one type to another related type, C++ provides the following
operators:

<dl>
  <dt><code>static_cast&lt;T *&gt;(U *)<br>static_cast&lt;T &amp;&gt;(U &amp;)</code></dt>
  <dd>
    <ul>
      <li>required if neither T nor U are polymorphic
      <li>permitted in other cases.
      <li>fails to detect erroneous casts of polymorphic pointers/references at runtime.
      <li>does not permit "cross casting"
      <li>inline function calls can be optimized away during compile time.
    </ul>
  </dd>
  <p>
  <dt><code>dynamic_cast&lt;T *&gt;(U *)<br>dynamic_cast&lt;T &amp;&gt;(U &amp;)</code></dt>
  <dd>
    <ul>
      <li>permitted if either T or U are polymorphic
      <li>prohibited in other cases.
      <li>throws exception on detecting erroneous casts of polymorphic pointers/references
          at runtime.
      <li>permits "cross casting"
      <li>cannot optimise inline virtual functions at compile time.
    </ul>
  </dd>
</dl>

These rules can make it difficult to use casting with a function template argument.
Consider the following example:

<pre><code>
#include &lt;boost/serialization/smart_cast.hpp&gt;

struct top {
};

struct base1 : public top {
    bool is_storable() const {
        return true;
    }
    virtual ~base1();
};

struct base2 {
    virtual ~base2();
};

struct derived1 :
    public base1
{
    derived1();
};

struct derived2 :
    public base1, 
    public base2
{
    derived2();
};

template&lt;class T&gt;
bool is_storable(T &amp;t){
    // what type of cast to use here?

    // this fails at compile time when T == base2
    // return static_cast&lt;base1 &amp;&gt;(t).is_storable();

    // this fails at compile time when T == top
    // otherwise it works but cannot optimize inline function call
    // return dynamic_cast&lt;base1 &amp;&gt;(t).is_storable();

    // this always works - and is guaranteed to generate the fastest code !
    return (boost::smart_cast_reference&lt;base1 &amp;&gt;(t)).is_storable();
}

int main(){
    derived1 d1;
    top &amp; t1 = d1;
    derived2 d2;
    base2 &amp; b2 = d2;

    bool result;
    result = is_storable(d1);   
    result = is_storable(d2);   
    result = is_storable(b2);
    result = is_storable(b2);
    result = is_storable(t1);
    return 0;
}
</code></pre>
The serialization library includes a mix of classes which use
both static polymorphism (<strong>CRTP</strong>) and dynamic
polymorphism via virtual functions.  <code style="white-space: normal">smart_cast</code> 
was written to address the more problematic manifestations of the
situation exemplified above.

<h3>Usage</h3>
The following syntax is supported:
<pre><code>
smart_cast&lt;Target *, Source *&gt;(Source * s);
smart_cast&lt;Target *&gt;(Source * s);
smart_cast&lt;Target &amp;, Source &amp;&gt;(Source &amp; s);
</code></pre>
Note that the above syntax doesn't include 
<pre><code>
smart_cast&lt;Target &amp; &gt;(Source &amp; s)
</code></pre>
but the same functionality is supported with the following special syntax
<pre><code>
smart_cast_reference&lt;Target &amp;&gt;(Source &amp; s)
</code></pre>

<h3>Requirements</h3>
<code style="white-space: normal">smart_cast</code> can be used only on compilers that support partial
template specialization or on types for which the
macro <code style="white-space: normal">
BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION(&lt;type&gt;)</code>
has been applied.

<hr>
<p><i>&copy; Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004. 
Distributed under the Boost Software License, Version 1.0. (See
accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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